Dopaminergic modulation of motor neuron activity and neuromuscular function in Drosophila melanogaster

Abstract

Dopamine is found in both neuronal and non-neuronal tissues in the larval stage of the fruit fly, Drosophila melanogaster, and functions as a signaling molecule in the nervous system. Although dopaminergic neurons in the central nervous system (CNS) were previously thought solely to be interneurons, recent studies suggest that dopamine may also act as a neuromodulator in humoral pathways. We examined both application of dopamine on intact larval CNS-segmental preparations and isolated neuromuscular junctions (NMJs). Dopamine rapidly decreased the rhythmicity of the CNS motor activity. Application of dopamine on neuromuscular preparations of the segmental muscles 6 and 7 resulted in a dose-responsive decrease in the excitatory junction potentials (EJPs). With the use of focal, macro-patch synaptic current recordings the quantal evoked transmission showed a depression of vesicular release at concentrations of 10 μM. Higher concentrations (1 mM) produced a rapid decrement in evoked vesicular release. Dopamine did not alter the shape of the spontaneous synaptic currents, suggesting that dopamine does not alter the postsynaptic muscle fiber receptiveness to the glutaminergic motor nerve transmission. The effects are presynaptic in causing a reduction in the number of vesicles that are stimulated to be released due to neural activity.

Introduction

Neuromodulators are known to function as important signaling molecules in animals, and can alter activity of the central and peripheral nervous systems [25], [42], [43]. One neuromodulator in particular, serotonin (5-HT), is involved in the behavioral expression of dominance and aggression in widely evolutionarily diverged species; the role of 5-HT in aggression has been explored in crustaceans [31], [45], [51] and humans [10], [11], [30], [50]. In Drosophila melanogaster, 5-HT modulates heartbeat [26], [33], and voltage dependence of delayed rectifier and Shaker potassium channels [24]. Dopamine and 5-HT have been identified in fly heads, which suggests that neural activity is regulated by these compounds.

Little is known about the neuromodulatory roles of dopamine. It has been shown to act as a neurotransmitter in Drosophila, modulating female sexual receptivity and habituation, a form of learning [36], [37]; however, it is also required for the normal development of both gonadal and other tissues [35]. We propose dopamine plays a functional role in behavioral modulation, neuroendocrine activity, and in development. Functional roles for dopamine in a variety of insects is well substantiated [21]; for example, the dopamine receptor densities in the brain of the honey bee are altered during development [29] and in relation to certain behaviors in bees [49]. The cloning and functional characterization of dopamine [22] and octopamine [23] receptors from Drosophila have been described. Localization and characterization of these receptors at the NMJ will hopefully be forthcoming.

To understand the neuromodulator effects of 5-HT and dopamine in D. melanogaster, we examined the synaptic efficacy of D. melanogaster motor neurons at the neuromuscular junctions of high- and low-output terminals during the third instar larva stage by recording evoked and spontaneous quantal currents for quantal analysis. With this approach, one can assess pre- as well as post-synaptic differences induced by the addition of particular neuromodulators. Our results suggest a functional role for dopamine as a neuromodulator at the neuromuscular junction.